1. Technical Field
The present invention relates to the field of metrology, and more particularly, to scatterometry overlay metrology.
2. Discussion of Related Art
Scatterometry overlay metrology tools use focus measurements to determine the degree of focus on the metrology targets on which overlay data is measured.
Kocher 1983 (Automated Foucault test for focus sensing, Applied Optics 22(12): 1887-1892) teaches focus measurements using a chopper wheel, a bicell photodetector, and an electronic phase detector to obtain a linear focus error voltage for an isolated point image, also known as knife edge focusing due to their utilization of the dependency of the direction of the knife-edge shadow motion (e.g., chopper wheel member) on the axial position of the edge relative to the focus.